In air-to-air refuelling, a tanker aircraft trails a refuelling hose, to the trailing end of which is attached a drogue assembly comprising a reception coupling and drogue canopy. The drogue canopy stabilises the refuelling hose trailing from the tanker aircraft, and provides a drag force.
A receiver aircraft is equipped with a refuelling probe which projects from the receiver aircraft. The refuelling probe comprises a nozzle disposed at the forward end of the refuelling probe and arranged to engage with the reception coupling. The reception coupling comprises an opening for receiving the refuelling probe.
On engagement of the refuelling probe with the reception coupling, the refuelling probe acts against the drag force provided by the drogue assembly. Following engagement a fuel flow path is established between the tanker aircraft and the receiver aircraft via the refuelling hose, the drogue assembly, and the refuelling probe.
The reception coupling comprises a fuel passage therethrough providing a channel for fuel for refuelling a receiver aircraft, and a valve for opening and closing the fuel passage to prevent fuel freely flowing out from the refuelling hose when the refuelling probe of a receiver aircraft is not engaged with the reception coupling. Conventional reception couplings comprise a poppet-type valve.
Fuel in the refuelling hose is pressurised to achieve a high flow rate of fuel to the receiver aircraft. The fuel pressure tends to act on a surface of the valve urging it towards a closed position.
On engagement of the refuelling probe with the reception coupling, the refuelling probe pushes against the valve with an engagement force to open the fuel passage. The engagement force must be sufficient to overcome the closing force on the valve caused by the fuel pressure. Engagement forces (loads) can therefore be high.
In some tanker refuelling systems the fuel pressure within the refuelling hose and the reception coupling can be reduced to reduce the engagement loads. However, this tends to lead to a lower flow rate of fuel to the receiver aircraft.
Some tanker refuelling systems are not capable of reducing the fuel pressure within the refuelling hose and the reception coupling, leading to high engagement loads, and requiring corresponding high relative speeds between the receiver aircraft and the tanker aircraft to ensure full engagement. This can cause issues with larger receiver aircraft due to the distance required by the aircraft to slow down following engagement.
For all aircraft, high engagement load (i.e. high relative engagement speed) is a constraint resulting in reduced precision and probability of full engagement.
The present invention is aimed at addressing such issues.